DLTS Study of Optoelectronic Devices in the Dynamic Regime
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DLTS Study of Optoelectronic Devices in the Dynamic Regime
S. Mil'shtein*, D. Tripp+ and A. Karakashian+ *Electrical Engineering Department +Department of Physics and Applied Physics University of Lowell, Lowell, MA 01854
ABSTRACT Every semiconductor device contains as grown and process induced defects even though the techniques to minimize the density of defects are known. Conventional DLTS methods are used to identify the trapping defects in the device. However, the activity of the traps could be suppressed or enhanced by the operating conditions of the device such as temperature, current through the device or light directed onto the device. We studied the DLTS spectra of silicon polycrystaline solar cells, as well as silicon and GaAs photodetectors in the dynamic regime, namely under light and at elevated temperature. Significant change in the resulting spectra were observed when compared to the spectra obtained by conventional DLTS methods. INTRODUCTION Deep Level Transient Spectroscopy (DLTS) is an indispensable tool in studies of defects in optoelectronic materials[I]. DLTS information on density of defects, their activation energy, and trapping cross sections, helps in understanding how the efficiency of an optoelectronic device depends on the presence of various defects. In solar cells, for example, the conditions always present during operation are intense generation of free carriers by light, elevated temperatures due to solar radiation and forward bias current through the p-n junction. These factors determine the dynamic regime of a solar cell which is very different from the regime of conventional DLTS tests (reverse bias of a junction, darkness, temperature scan from room temperature down to liquid nitrogen).
Mat. Res. Soc. Symp. Proc. Vol. 209. C1991 Materials Research Society
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The goal of the study of optoelectronic devices is to demonstrate the feasibility of a new approach called DLTS in the dynamic regime[2]. C-V profiling and DLTS measurements were performed using a DLTS-4600 computerized spectrometer (produced by Biorad of Cambridge, MA.) EXPERIMENT A Schottky diode made by vacuum deposition of Ag on n-silicon with a window etched in the Schottky contact was tested under both dark and tungsten source illumination. The illumination produced twice as many free carriers in the device by over the gap transitions as measured by C-V profiling. In turn the intensity of trapping, was increased by a factor of two. Fig. 1 presents the activity of the traps in the silicon photodetector. The solid line peak (trapping under illumination) is two times higher than the cross marked peak (trapping in the dark). The DLTS spectrum taken in the dark revealed three electron traps with activation energies of 315meV, 190meV, and 175meV. The peak height at 280 0 K is unchanged by the
Temperature (CKi
Fig. 1 DLTS of Si photodetector plotted for window rate 1l00s-1 *DLTS in the dark. -DLTS under illumination. (The crosses are not data points but are used to distinguish curves.)
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illumination, but the two
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